1. Field of the Invention
The present invention relates to a direct two-step procedure for the replacement of the pyranose anomeric 10-OH group in dihydroartemisinin by various carbon nucleophiles without the destruction of the trioxane peroxide bond. The present invention further relates to the formation of monomeric and dimeric C-10 carbon-substituted (rather than oxygen substituted) derivative of the trioxane 10-deoxoartemisinin which demonstrate potent and potentially therapeutically valuable antimalarial, and antiproliferative and antitumour activities.
2. Description of the State of Art
Each year approximately 200-300 million people experience a malarial illness and over 1 million individuals die. In patents with severe and complicated disease, the mortality rate is between 20 and 50%.
Plasmodium is the genus of protozoan parasites which is responsible for all cases of malaria and Plasmodium falciparum is the species of parasite that is responsible for the vast majority of fatal malaria infections. Malaria has traditionally been treated with quinolines such as chloroquine, quinine, mefloquine, and primaquine and with antifolates such as sulfadoxine-pyrimethamine. Unfortunately, most P. falciparum strains have now become resistant to chloroquine, and some, such as those in Southeast Asia, have also developed resistance to mefloquine and halofantrine; multidrug resistance is developing in Africa also.
The endoperoxides are a promising class of antimalarial drugs which may meet the dual challenges posed by drug-resistant parasites and the rapid progression of malarial illness. The first generation endoperoxides include artemisinin and several synthetic derivatives, discussed in further detail below.
Artemisia annua L., also known as qinghao or sweet wormwood, is a pervasive weed that has been used for many centuries in Chinese traditional medicine as a treatment for fever and malaria. Its earliest mention, for use in hemorrhoids, occurs in the Recipes for 52 Kinds of Diseases found in the Mawangdui Han dynasty tomb dating from 168 B.C. Nearly five hundred years later Ge Hong wrote the Zhou Hou Bei Ji Fang (Handbook of Prescriptions for Emergency Treatments) in which he advised that a water extract of qinghao was effective at reducing fevers. In 1596, Li Shizhen, the famous herbalist, wrote that chills and fever of malaria can be combated by qinghao preparations. Finally, in 1972, Chinese chemists isolated from the leafy portions of the plant the substance responsible for its reputed medicinal action. This crystalline compound, called qinghaosu, also referred to as QHS or artemisinin, is a sesquiterpene lactone with an internal peroxide linkage.
Artemisinin is a member of the amorphane subgroup of cadinenes and has the following structure (I): ##STR2## Artemisinin or QHS was the subject of a 1979 study conducted by the Qinghaosu Antimalarial Coordinating Research Group involving the treatment of 2099 cases of malaria (P. vivax and P. falciparum in a ratio of about 3:1) with different dosage forms of QHS, leading to the clinical cure of all patients. See, Qinghaosu Antimalarial Coordinating Research Group, Chin. Med. J., 92:811 (1979). Since the time artemisinin has been used successfully in many thousand malaria patients throughout the world including those infected with both chloroquine-sensitive and chloroquine-resistant strains of P. falciparum. Assay of artemisinin against P. falciparum in vitro revealed that its potency is comparable to that of chloroquine in two Hanian strains (Z. Ye, et al., J. Trad. Chin. Med., 3:95 (1983)) and of mefloquine in the Camp (chloroquine-susceptible) and Smith (chloroquine-resistant) strains, D. L. Klayman, et al., J. Nat. Prod., 47:715 (1984).
Although artemisinin is effective at suppressing the parasitemias of P. vivax and P. falciparum, the problems encountered with recrudescence, and the compound's insolubility in water, led scientists to modify artemisinin chemically, a difficult task because of the chemical reactivity of the peroxide linkage which is believed to be an essential moiety for antimalarial activity.
Reduction of artemisinin in the presence of sodium borohydride results in the production of dihydroartemisinin (II-1) or DHQHS, (illustrated in structure II below), in which the lactone group is converted to a lactol (hemiacetal) function, with properties similar to artemisinin. Artemisinin in methanol is reduced with sodium borohydride to an equilibrium mixture of .alpha.- and .beta.-isomers of dihydroartemisinin. The yield under controlled conditions is 79% (artemisinin, 0.85M with NaBH.sub.4 6:34M, 7:5 equivalents in methanol, 12 L at 0-5.degree. C. for about 3 hours followed by quenching with acetic acid to neutrality at 0-5.degree. C. and dilution with water to precipitate dihydroartemisinin), A. Brossi, et al., Journal of Medicinal Chemistry, 31:645-650 (1988). Using dihydroartemisinin as a starting compound a large number of other derivatives, such as: ##STR3## artemether (compound II-2), arteether (II-3), sodium artesunate (II-4), artelinic acid (II-5), sodium artelinate (II-6), dihydroartemisinin condensation by-product (II-7) and the olefinic compound, structure III: ##STR4## have been produced.
Artemether (II-2) is produced by reacting .beta.-dihydroartemisinin with boron trifluoride (BF.sub.3) etherate or HCl in methanol:benzene (1:2) at room temperature. A mixture of .beta.- and .alpha.-artemether (70:30) is obtained, from which the former is isolated by column chromatography and recrystallized from hexane or methanol, R. Haynes, Transactions of the Royal Society of Tropical Medicine and Hygiene, 88(1): S1/23-S1/26 (1994). For arteether (II-3), (Brossi, et al., 1988), the .alpha.-isomer is equilibrated (epimerized) to the .beta.-isomer in ethanol:benzene mixture containing BF.sub.3 etherate. Treatment of dihydroartemisinin with an unspecified dehydrating agent yields both the olefinic compound, (III), and the dihydroartemisinin condensation by-product (II-7), formed on addition of dihydroartemisinin to (III), M. Cao, et al., Chem. Abstr., 100:34720k (1984). Until recently, the secondary hydroxy group in dihydroartemisinin (II-1) provided the only site in an active artemisinin-related compound that had been used for derivatization. See B. Venugopalan, "Synthesis of a Novel Ring Contracted Artemisinin Derivative," Bioorganic & Medicinal Chemistry Letters, 4(5):751-752 (1994).
The potency of various artemisinin-derivatives in comparison to artemisinin as a function of the concentration at which the parasitemia is 90 percent suppressed (SD.sub.90) was reported by D. L. Klayman, "Qinghaosu (Artemisinin): An Antimalarial Drug from China," Science 228:1049-1055 (1985). Dr. Klayman reported that the olefinic compound III is inactive against P. berghei-infected mice, whereas the dihydroartemisinin condensation by-product (II-7) has an SD.sub.90 of 10 mg/Kg in P. berghei-infected mice. Thus, the dihydroartemisinin ether dimer proved to be less potent than artemisinin, which has an SD.sub.90 of 6.20 mg/Kg. Following, in order of their overall antimalarial efficacy, are the three types of derivatives of dihydroartemisinin (II-1) that have been produced: (artemisinin)&lt;ethers (II, R=alkyl)&lt;esters [II, R.dbd.C(.dbd.O)-alkyl or -aryl]&lt;carbonates [II, R.dbd.C(.dbd.O)O-alkyl or -aryl].
Other rational design of structurally simpler analogs of artemisinin has led to the synthesis of various trioxanes, some of which possess excellent antimalarial activity. Posner, G. H., et al., reported the chemistry and biology of a series of new structurally simple, easily, prepared, racemic 1,2,4-trioxanes (disclosed in U.S. Pat. No. 5,225,437 and incorporated herein by reference) that are tricyclic (lacking the lactone ring present in tetracyclic artemisinin I) and that are derivatives of trioxane alcohol IV: ##STR5## having the relative stereochemistry shown above. Especially attractive features of trioxane alcohol IV are the following: (1) is straightforward and easy preparation from cheap and readily available starting materials, (2) its availability of gram scale, and (3) its easy one-step conversion, using standard chemical transformations, into alcohol derivatives such as esters and ethers, without destruction of the crucial trioxane framework. See, Posner, G. H. et al., J. Med. Chem., 35:2459-2467 (1992), incorporated herein by reference. The complete chemical synthesis of artemisinin and a variety of other derivatives is reviewed by Sharma, R. P., et al., Heterocycles, 32(8):1593-1638 (1991), and is incorporated herein by reference.
Metabolic studies by Baker, et al., demonstrated that .beta.-arteether (II-3), one of the antimalarial derivatives discussed previously, was rapidly converted by rat liver microsomes into dihydroartemisinin (II-1). See Baker, J. K., et al., Biol. Mass. Spect., 20:609-628 (1991). This finding and the fact that the most effective artemisinin derivatives against malaria have been ethers or esters of (II-1) suggest that they were prodrugs for (II-1). The controlled slow formation of (II-1), however, is not desirable in view of its short half-life in plasma (less than two hours) and relatively high toxicity.
The successful synthesis of anticancer and antiviral drugs by replacing a carbon-nitrogen bond in nucleosides by a carbon-carbon bond (C-nucleosides) prompted the preparation of several 10-alkyldeoxoartemisinins, V: ##STR6## wherein R is 1-allyl, propyl, methyl, or ethyl. Typically, these syntheses involved five or six steps and the reported yields were only about 12 percent. See, Jung, M., et al., Synlett., 743-744 (1990); and Haynes, R. K., et al., Synlett., 481-484 (1992).
Over the past thirty years only a few drugs isolated from higher plants have yielded clinical agents, the outstanding examples being vinblastine and vincristine from the Madagascan periwinkle, Catharnathus roseus, etoposide, the semi-synthetic lignan, from Mayapple Podophyllum peltatum and the diterpenoid taxol, commonly referred to as paclitaxel, from the Pacific yew, Taxus brevifolia. Of these agents, paclitaxel is the most exciting, recently receiving approval by the Food and Drug Administration for the treatment of refractory ovarian cancer. Since the isolation of artemisinin, there has been a concerted effort by investigators to study other therapeutic applications of artemisinin and its derivatives.
National Institutes of Health reported that artemisinin is inactive against P388 leukemia. See NCI Report on NSC 369397 (tested on Oct. 25, 1983). Later anticancer studies that have been conducted on cell line panels consisting of 60 lines organized into nine, disease-related subpanels including leukemia, non-small-cell lung cancer, colon, CNS, melanoma, ovarian, renal, prostate and breast cancers, further confirm that artenisinin displays very little anticancer activity. A series of artemisinin-related endoperoxides were tested for cytotoxicity to Ehrlich ascites tumor (EAT) cells using the microculture tetrazolum (MTT) assay, H. J. Woerdenbag, et al. "Cytotoxicity of Artemisinin-Related Endoperoxides to Ehrlich Ascites Tumor Cells," Journal of Natural Products, 56(6):849-856 (1993). The MTT assay, used to test the artemisinin-related endoperoxides for cytotoxicity, is based on the metabolic reduction of soluble tetrazolium salts into insoluble colored formazan products by mitochondrial dehydrogenase activity of the tumor cells. As parameters for cytotoxicity, the IC.sub.50 and IC.sub.80 values, the drug concentrations causing respectively 50% and 80% growth inhibition of the tumor cells, were used. Artemisinin (I) had an IC.sub.50 value of 29.8 .mu.M. Derivatives of dihydroartemisinin (II-1) being developed as antimalarial drugs (artemether (II-2), arteether (III-3), sodium artesunate (II-4), artelinic acid (II-5) and sodium artelinate (II-6)), exhibited a somewhat more potent cytotoxicity. Their IC.sub.50 values ranged from 12.2 .mu.M to 19.9 .mu.M. The dihydroartemisinin condensation by-product dimer (II-7), disclosed previously by M. Cao, et al., 1984, was the most potent cytotoxic agent, its IC.sub.50 being 1.4 .mu.M. At this drug concentration the condensation by-product (II-7) is approximately twenty-two times more cytotoxic than artemisinin and sixty times more cytotoxic than dihydroartemisinin (II-1), the parent compound.
While artemisinin and its related derivatives (II-1-6) discussed above demonstrated zero to slight antiproliferative and antitumor activity, it has been discovered that a class of artemisinin dimer compounds exhibits antiproliferative and antitumor activities that are, in vitro, equivalent to or greater than known antiproliferative and antitumor agents. See, U.S. Pat. No. 5,677,468 incorporated herein by reference. Unfortunately, while the in vitro results of these artemisinin compounds are encouraging these compounds do not appear to have significant antitumor activity on the treatment of tumor cells in mice.
There is still a need, therefore, to develop a more efficient method for the formation of hydrolytically more stable C-10 carbon-substituted deoxoartemisinin compounds and structural analogs thereof having antimalarial, and antiproliferative and antitumor activities that are equivalent to or greater than those of known antimalarial, and antiproliferative and antitumor agents, respectively.